In a vehicle of four-wheel drive type in which the front and rear wheels are driven by the same engine, there is a slight difference between the effective radii of the tires of the front and rear wheels or there occurs a difference between the paths of rolling movement of the tires of the front and rear wheels during running along a curve. As a result, the tires tend to slip during running of the vehicle, and an excessive force tends to act upon the drive system of the vehicle. It is therefore necessary to provide effective means for preventing impartation of such an undesirable force to the drive system of the vehicle.
For this purpose, in a vehicle of full-time four-wheel drive type in which all of the four wheels are necessarily driven at the same time, there has been provided a third differential unit commonly called a center differential unit so as to permit transmission of the drive force of the engine to all the wheels even when a rotation speed difference occurs between a first rotary shaft transmitting the drive force to the front wheels and a second rotary shaft transmitting the drive force to the rear wheels. However, the vehicle of full-time four-wheel drive type is disadvantageous from the aspects of weight, size and cost when compared with a vehicle of part-time four-wheel drive type in which all of the four wheels are not necessarily driven at the same time. Also, owing to the capability of the differential rotation of the front and rear wheels, the four-wheel drive may not always be achieved when the four-wheel drive is required, and, in order to ensure the four-wheel drive, a differential locking mechanism is additionally required. Thus, the vehicle of full-time four-wheel drive type has been defective among others in that its power transmission system is complex in structure.
In connection with such a vehicle of full-time four-wheel drive type, a method is disclosed in Japanese Patent Application Laid-open No. 58-20521 (1983). According to the disclosed method, the center differential unit is replaced by a wet type multi-disk clutch disposed in the drive coupling portion, and such a clutch is caused to slide during running of the vehicle along a curve thereby absorbing the rotation speed difference between the front and rear wheels. However, the proposed method has been defective among others in that the wet type multi-disk clutch tends to be damaged by the heat generated due to slippage and that the clutch has a limited torque transmission capacity.
The center differential unit is not usually provided in the vehicle of part-time four-wheel drive type. Accordingly, the driver of the vehicle of this type must make necessary manipulation to drive the two wheels only among the four wheels when a tight corner braking phenomenon peculiar to the four-wheel drive may occur during running of the vehicle along a curve. Therefore, the vehicle of part-time four-wheel drive type has been defective in that the driver must make complicated manipulation for driving the vehicle.